Concrete

is a lightweight, web-based editor for Domain Specific Languages (DSLs)
is setup for a specific DSL, knows the language commands and provides Auto Completion
permanently checks the consistency with the DSL and provides Error Annotations
can give language elements almost any look expressible with HTML and CSS
is a Javascript widget which can be embedded into virtually any web application
exchanges data with a backend via the widely used JSON format
is based on Prototype and uses Scriptaculous


http://concrete-editor.org/#documentation